Taylor Swift, Carly Simon share 'You're So Vain' duet on Red Tour

Taylor Swift welcomed guest Carly Simon on stage to sing the latter's 1973 hit "You're So Vain" to a stadium of concertgoers in Massachusetts.

So does this count as her giving Generation Y a healthy dose of the 1970s and the original breakup anthem?

On Saturday, clas in red shorts and a striped top, Swift treated her fans to a duet during the Foxborough, Mass., stop of her Red Tour.

"Before I bring out my special guest, I have this question that I've always, always had," Swift announced. "I think a lot of you have probably had this question too, always wondered what the answer is. Who is this song, 'You're So Vain' by Carly Simon, written about?"

The crowd just about lost it when Simon emerged onstage. And so the pair sang the vengeful song, to their delight.

"Ladies and gentleman, give it up for one of your own, Carly Simon!" Swift shouted when they were done.

While neither Swift nor Simon addressed Swift's original question (who it's about), the audience didn't seem to mind. Mick Jagger, Kris Kristofferson, James Taylor, Warren Beatty and the former Cat Stevens are among some of the potential subjects Simon, 68, could have written the song about.

The 23-year-old singer-songwriter later took to Twitter to share her squeal-worthy news.

"Got to sing 'You're So Vain' with Carly Simon at Gillette Stadium tonight!!!!!" she wrote. "Unreal. I could not possibly look up to her more."

Swift later said that a backstage interview with herself and Simon would air Monday on "Good Morning America," calling the performance "one of the most special nights of the tour."

"Once we get out there and we have 55,000 people in a sing-along it's just going to be like karaoke night in the stadium. Every girl out there will be thinking of someone when we're singing," Swift said.

"When I heard 'You're So Vain' I just thought that is the best song that has ever been written," she added. "That is the most direct way anyone has ever addressed a breakup. It's amazing."

"It's a song that really sort of developed that style of mine," Simon said. "A lot of it was not knowing that I developing that style because lot of things happen by accident.

"It was just so wonderful that I just want to do it again," Simon said of the performance. "[Taylor] was so great, and she really led me through my own song, and she was just so loving of the whole thing, and we loved each other and we moved together in a part. It was dramatic, it was great."

As for who it was written about, Simon shared her secret with Swift but has forbidden her from telling.